Test admin handler: create a unique class-attribute `sample_request.

Why duplicating code everywhere when we do have this cool thing called ENVIRONMENTS?
Test Admin Handler: spotting unicode bug in put.

Apparently the server accepts non-unicode stuff and throws out str(), not
unicode(). This must be fixed.
Check globaleaks.tests.handlers.test_admin.TestAdminFieldsInstance.test_put for
more informations.